1. Field of the Invention
The present invention relates to an exposure apparatus and device manufacturing method.
2. Description of the Related Art
An exposure apparatus is used in the manufacture of a micropattern employed in various types of devices such as semiconductor device and liquid crystal devices, and micromechanics. As the devices and the like manufactured by the exposure apparatus shrink in feature size, the exposure wavelength is becoming short from that (248 nm) of KrF to that (193 nm) of ArF. Furthermore, attempts are being made to extend the limitations of photolithography by introducing the immersion exposure technique to the ArF wavelength, so that an equivalent wavelength of 134 nm is achieved.
Micropatterning is one of the most significant factors that support the dynamics of the semiconductor industry. Ever since the days when a resolution of 0.25 μm was required of a 256M DRAM, the micropatterning generation is rapidly changing to 180 nm, 130 nm, and even 100 nm.
Lithography up to i-line (365 nm) has not employed a resolution equal to or less than the wavelength. Although KrF has a wavelength of 248 nm, however, it has been employed for a line width of 180 nm and furthermore 130 nm.
Hence, an epoch has finally arrived when a resolution equal to or less than the wavelength is put into practical use by making good use of the development in resist and the accomplishment in the ultrahigh resolution technologies and the like. A line width ⅓ the wavelength in a line-and-space pattern is coming into view if various types of ultrahigh resolution technologies are fully utilized.
In this situation, line width control is currently the most significant issue. The committee of ITRS (International Technology Roadmap for Semiconductors) which draws a technology roadmap for the semiconductor industry continues to propose specifications that future semiconductor elements should satisfy.
According to the ITRS committee, among various items of lithography, line width control (CD control) will encounter its limitations the earliest. The items contributing to line width control vary to include the exposure apparatus, reticle, and process. How to decrease the values of the respective items to the limits to improve the controllability is the major issue.
One of the major factors that improve the line width controllability is the projection optical system of the exposure apparatus, and particularly the aberration of the projection optical system is one significant item. Hence, how to decrease aberration is always a major issue, and many efforts have been made to achieve this. Reviewing the past ten years, aberration decrease has advanced a great deal. For example, line width controllability with a value falling well below 10 mλ, when expressed by a Zernike series expanded to include as many terms as down to the 36th term which serves as a main value, has been realized.
Note that the aberration described above refers to the aberration unique to the projection optical system due to the design value, errors in assembly and adjustment, nonuniformity in glass material, and the like. As is well known, the semiconductor manufacture employs the exposure method of illuminating a reticle pattern and transferring and projecting light transmitted through the reticle pattern onto a wafer surface through a projection optical system. The glass material used in the projection optical system has very high transmittance for the wavelength of light employed for exposure but slightly absorbs exposure light so that its temperature rises. This temperature rise changes the refractive index of the glass material and deforms the surface shape to generate aberration. This aberration is called exposure aberration.
Exposure aberration must be decreased because it also degrades various types of optical characteristics including line width controllability. Among methods of decreasing exposure aberration, the most basic decreasing methods include a method of correcting an error in focal position caused by exposure aberration by moving the wafer stage, and a method of correcting fluctuations in imaging magnification by moving some of the lenses.
The amount and characteristics of exposure aberration change depending on the exposure method and exposure conditions, and accordingly exposure aberration must be corrected in accordance with the individual characteristics. For example, in a step-and-repeat exposure apparatus or a so-called stepper, that portion of a reticle through which light is transmitted is close to a square, which is almost rotationally symmetric if the density in pattern arrangement does not largely change. Therefore, a highly symmetric correction method of heating the periphery of the lens entirely, as described in Japanese Patent Laid-Open No. 5-347239, can be employed in addition to focal position misalignment correction and imaging magnification correction.
In an exposure apparatus employing the step and scan method or a so-called scanner, the reticle side light-transmitting region is rectangular (typically, the ratio in length of the long side to the short side is larger than 3). Accordingly, that portion of each of at least lenses near the reticle and wafer which is irradiated with light is almost rectangular. Considering the fact that light absorption and heat generation occur in this region, changes in refractive index and surface shape also occur near the irradiation region. Hence, a rotationally asymmetric component, mainly astigmatism, which is not large in the stepper, occurs in the scanner. To cope with this, methods are proposed such as a method of enabling partial lens heating, as in Japanese Patent Laid-Open No. 8-8178, and a method of employing light having a wavelength different from that used for exposure so that the pattern on the reticle-side image plane becomes almost rotationally symmetric, as in Japanese Patent Laid-Open No. 10-50585.
Of exposure conditions that change the exposure aberration, the illumination condition is the most important. Considering a case in which no pattern is arranged on the reticle and light is transmitted through the reticle completely, an effective light source given as an illumination condition forms an image near the pupil of the projection optical system. If taking into account only 0th-order diffracted light which usually contributes to light transmission the most among light diffracted by the reticle, the same applies even to a case in which a pattern is arranged on the reticle.
As a technique effective for improving the resolution, an oblique-incidence illumination method (also called deformed illumination as well) is known. According to this method, the light source is arranged at a position away from an optical axis which is the rotationally symmetric axis of the projection optical system. The shape of this light source includes a zone type, quadrupole type, dipole type, and the like. A lens near the pupil of the projection optical system focuses light from an effective light source and absorbs light to generate heat. Particularly, when the light source is of the dipole type or the like, the region of the lens irradiated with light is limited, thus causing astigmatism. Regarding this problem, Japanese Patent Laid-Open Nos. 8-8178 and 2005-311020 disclose a method of partially heating a lens in question to form an almost rotationally symmetric temperature distribution and correcting the remaining aberration. Also, Japanese Patent Laid-Open No. 10-64790 discloses a method of arranging a light source, having a wavelength different from the exposure wavelength, at a position not used for oblique-incidence illumination.
Currently, the state-of-the art semiconductor element manufacture employs a scanner, and quadrupole illumination and dipole illumination are employed as a standard practice to improve the resolution. Accordingly, rotationally asymmetric exposure aberration occurs due to the influence of lenses near the reticle and wafer and a lens near the pupil of the projection optical system. Hence, demands have arisen to correct the lenses near the reticle and wafer and the lens near the pupil of the projection optical system simultaneously. In addition, line width control requires higher accuracy so that it can correct and decrease even an aberration component of a higher order (e.g., the 17th term or more when expressed by a Zernike series) than in the conventional case. In view of these demands, to correct the lenses near the reticle and wafer and the lens near the pupil of the projection optical system simultaneously, a correction mechanism may be mounted for each lens, so that each lens is controlled independently.
Japanese Patent Laid-Open Nos. 10-50585 and 2005-311020 described above are based on this idea, but have limitations that, for example, correction mechanisms must be mounted for a plurality of lenses, and if the lenses are arranged close to each other as in the projection optical system of a semiconductor exposure apparatus, a temperature rise for the purpose of correction needs to be able to be added for only the peripheral portions of the lenses. If light must be guided by oblique incidence, as in the latter case, when reflected light or transmitted light irradiates a lens other than the lens to be corrected, a holding portion for such a lens, or a lens barrel of such a lens, it may also cause exposure aberration. Based on these possibilities, exposure aberration correction must be performed in the entire projection optical system using a simple arrangement.